Power Converter – Watts, Kilowatts and Horsepower

Power units and their definitions

Power is energy per unit of time. The SI unit is the watt (W): one joule per second. The converter uses these values:

• 1 kilowatt (kW) = 1,000 W; 1 megawatt (MW) = 1,000,000 W
• 1 mechanical horsepower (hp) = 745.7 W, based on 550 foot-pounds per second
• 1 metric horsepower (PS, CV, pk) = 735.5 W, based on 75 kilogram-force metres per second
• 1 BTU per hour (BTU/h) = 0.2931 W

Horsepower: hp versus PS

Two horsepower definitions are in common use. Car manufacturers in Europe usually quote metric horsepower (PS in German, CV in French and Italian, pk in Dutch), while US and UK figures use mechanical horsepower (hp or bhp). A metric horsepower is about 1.4% smaller, so a 150 PS engine produces 110.3 kW or 147.9 hp. Vehicle registration documents in the EU list power in kilowatts, which avoids the ambiguity.

Heating and cooling: BTU per hour

Air conditioners and heaters in North America are rated in BTU per hour, often shortened to just “BTU”. A 12,000 BTU/h air conditioner – one “ton” of cooling – delivers about 3.5 kW of cooling power. A 9,000 BTU/h unit is about 2.6 kW. Use the AC size and BTU heating calculators to estimate how much capacity a room needs.

Electrical power

Household appliances list their power in watts: a kettle draws around 2,000–3,000 W, a laptop 30–100 W and an LED bulb 5–10 W. Multiply power by time to get energy: a 2 kW kettle used for six minutes uses 0.2 kWh. To convert that energy into joules or BTU, use the energy converter.

Solar panels and generators

Solar panels are rated in watts peak (Wp), the output under standard test conditions, and home installations are described in kilowatts peak. Generators and inverters list both a continuous and a peak rating in watts or kilowatts; size them for the continuous power of everything that runs at the same time.

Sources

Conversion factors follow the SI definitions in the BIPM SI Brochure and the exact factors for US customary and imperial units in NIST Special Publication 811.